1. Field of the Invention
The invention, in general, relates to a novel method of applying a zinc sulfide buffer layer to a semiconductor substrate by chemical bath deposition, and, more particularly, to the absorber layer of a thin-film solar cell. The method may also be applied, for instance, in the production of II-IV “superstrat” thin-film solar cells and chalcopyrite “superstrat” thin-film solar cells.
2. The Prior Art
Currently, great efforts are being made to develop cost-efficient solar cells. In this respect, chalcopyrite thin-film solar cells represent the latest state of development. In this connection, layers of copper and indium are applied to a substrate. Gallium may be added optionally. The gallium is then reacted with selenium and/or sulfur (CIS, CIGS or CIGSSe, generally CIS solar cells). Among the thin-film technologies, the CIS technology is especially interesting because of its environmental compatibility and because of its low costs.
The solar cells are produced by initially applying, usually in a high vacuum, an electrically conductive layer, usually a sputtered layer of molybdenum, onto float glass or soda lime glass followed by CIS, CIGS or CIGSSe of a thickness of less than 3 μm. As a rule, a buffer layer of cadmium sulfide is applied to this absorber layer. The cover layer then is a transparent and conductive layer, the so-called window layer. For producing modules, the absorber is applied to a structured glass-molybdenum substrate and is also structured following the application of the buffer layer, resulting in an electrical connection. By contacting and framing the entire structure a thin-film solar module results which serves to convert light energy into electrical power.
Because of the disadvantages of glass as a substrate various efforts have also been made to use flexible materials instead of glass. Thus, solar modules are known in which a metallic band is used as the substrate. Plastic foils have also become known for the CIS deposition. An electrolytic application of the CIS layer has also been suggested.
For such thin-film solar cells to operate efficiently, a buffer layer is required between the absorber layer (e.g. Cu(InaGab)(SxSey) and the window layer (i-ZnO/ZnO:Ga). It protects the surface from damage which may result from the application of the window layer. Furthermore, it provides for surface-doping of the absorber layer as well as for passivation of the surface to yield lower surface conditions. The three usual processes, viz. sputtering, vapor deposition or (electro-)chemical deposition are used to apply this very thin buffer layer. Sputtering and vapor deposition both require considerably complex apparatus. Chemical bath deposition (CBD) up to now precipitating cadmium sulfide (CdS) has so far been most desirable. Moreover, below 50 nm, only CBD results in a thin homogenous and uniform cover.
Since cadmium is an extremely toxic heavy metal attempts have been made to find alternatives. Thus, instead of cadmium sulfide, zinc sulfide (ZnS) may be used as well (see, e.g. Nakada et al., Polycrystalline (Cu(In,Ga)Se2 Thin Film Solar Cells with Chemically Deposited Sulfide (CdS, ZnS) Buffer Layers; 2nd World Conference and Exhibition on Photovoltaic Solar Energy Conversion, 6-10 Jul. 1998, Vienna.
However, chemical bath deposition processes using zinc sulfide have good deposition rates only when simultaneously using hydrazine. In such a process, zinc sulfide is precipitated from a solution of zinc acetate, thiourea, ammonia and hydrazine, see e.g. Neve et al., ERDA Analysis of ZnSx(OH)y Thin Films Obtained by Chemical Bath Deposition, Mat. Res. Symp. Proc. Vol. 668, 2001 Materials Research Society, H5.3.1- H5.3.6 or Ennaoui et al., Highly Efficient Cu(Ga,In)(S,Se)2 Thin Film Solar Cells with Zinc-Compound Buffer Layers, Thin Solid Films 431-432 (2003) pp. 335-339. According to the tests by Neve et al. ammonia may be left out; however, hydrazine is absolutely necessary. Since, however, hydrazine is classified as toxic and carcinogenic, this process offers not much of an ecological advantage. See Herrero et al. Journal of Electrochemical Society, Vol. 142, No. 3, 1995, S. 764-770.